Polypropylene-Based Monofilaments With Improved Properties

ABSTRACT

The invention relates to polypropylene-based monofilaments having improved properties, especially an improved abrasion resistance, to a process for manufacturing these monofilaments and to their use. The invention relates more particularly to polypropylene-based monofilaments obtained from a polypropylene having a melt flow index (MFI) at 230° C./2.16 kg of less than or equal to 1.9 g/10 min.

The invention relates to polypropylene-based monofilaments having improved properties, especially an improved abrasion resistance, to a process for manufacturing these monofilaments and to their use. The invention relates more particularly to polypropylene-based monofilaments obtained from a polypropylene having a melt flow index (MFI) at 230° C./2.16 kg of less than or equal to 1.9 g/10 min.

Two-dimensional textile articles made of polypropylene are of great interest as chemically and mechanically resistant filtration means for filtration in the chemical, pharmaceutical and food industries.

Monofilaments made of pure polypropylene have the drawback of forming a great deal of dust as a result of the low resistance of this substance to abrasion during the weaving process. The problem of abrasion is also known in the case of other thermoplastics. Thus EP-A2-0 784 107 mentions melt-spun, polyamide, polyester or polypropylene monofilaments that are intended for paper machine forming fabrics and for strimmer lines. According to the above patent, abrasion-resistant monofilaments are obtained using 70 to 99% by weight of fibre-forming polymer and 1 to 30% by weight of a maleic-anhydride-modified polyethylene/polypropylene rubber and other additives. However, the examples are limited, as regards the fibre-forming polymer, to the polyamide nylon-6 and to polyethylene terephthalate, and also to a PA-6,6/PA-6 copolyamide. The spinning rates are not mentioned.

A process for manufacturing polypropylene multifilaments intended for textile applications is also known from EP-A-1 059 370. As starting material, an isotactic polypropylene catalysed by a metallocene is used that has a melt flow index greater than 19 g per 10 minutes. FOY (Fully Oriented Yarn) multifilaments of 10 dpf (denier per filament) [0.03953 mm diameter] and POY (Partially Oriented Yarn) multifilaments of 2 dpf (denier per fibre) [0.01768 mm diameter] have been described. As regards the yarns manufactured, only general indications are given. Monofilaments are not described.

Applications WO 02/086207 and WO 03/048434 describe polypropylene monofilaments, the polypropylene including additives. The polypropylene employed for manufacturing the monofilaments has a melt flow index of greater than or equal to 6 g per 10 minutes. These monofilaments have correct abrasion resistance properties.

One problem of the invention consists in how to provide polypropylene-based monofilaments that are more abrasion resistant than the monofilaments known from the prior art. Another problem of the invention consists in how to obtain polypropylene-based monofilaments that exhibit better abrasion resistance during weaving.

Yet another problem consists in how to provide for the use of monofilaments possessing good abrasion resistance for the manufacture of two-dimensional high-performance articles, especially those intended for filtration.

The problems are solved according to the invention in particular by the use, for the manufacture of the monofilaments, of a polypropylene having a melt flow index (MFI) at 230° C./2.16 kg of less than or equal to 1.9 g/10 min.

The polypropylenes conventionally used in the manufacture of monofilaments are isotactic polypropylenes.

The monofilaments of the invention advantageously have an abrasion of less than or equal to 0.014%.

The abrasion of the monofilaments is determined by gravimetric measurement of the amount of fluff produced during the manufacture of a 300 m length of woven fabric, using the method described below in the experimental part. The abrasion value corresponds to the result of the formula described below in the experimental part, calculated for a woven fabric length of 300 m.

It is particularly advantageous for the monofilaments to have an abrasion of less than 0.014%. This is because, when the abrasion is greater than 0.014%, irregularities may be produced during weaving, due to too rapid fouling of the reeds. This may shorten the time intervals between cleaning operations, thereby reducing the productivity of the loom.

The monofilaments of the invention preferably have an abrasion of less than or equal to 0.01%.

It is appropriate for the monofilaments to possess a strength of at least 50 cN/tex for an elongation corresponding to the maximum tensile stress (elongation at break) at least equal to 20%. This is because a strength of less than 50 cN/tex has the drawback of increasing the number of yarn breakages during the weaving process.

The monofilaments of the invention advantageously have a mechanical constant of greater than or equal to 285 cN/tex.

According to one particular embodiment of the invention, the monofilaments include an additive. Advantageously, the monofilaments include 0.01 to 20% by weight of additive relative to the weight of polypropylene.

Advantageously, it is appropriate to use as additive a polypropylene/polyethylene copolymer having a melting point ≧140° C. Advantageously, this additive is present in a proportion of 1 to 20% by weight relative to the weight of polypropylene.

The invention also relates to a process for manufacturing. the monofilaments of the invention, which comprises the steps of extrusion/melt-spinning, drawing and winding of the polypropylene.

The polypropylene-based monofilaments of the invention may be obtained using conventional spinning methods. They are generally obtained by extrusion/melt-spinning, cooling in a water bath when the monofilament titre is greater than or equal to 0.05 mm, drawing and winding. When the monofilaments of the invention include an additive, the additive is generally added to the polypropylene upstream of an extruder, before the spinning.

The monofilaments according to the invention are particularly suitable for the manufacture of two-dimensional high-performance articles, and especially for the manufacture of such articles in the field of filtration. These articles, when they are intended for filtration, especially in the chemical, pharmaceutical and food industries, are preferably woven fabrics.

The invention will now be described in greater detail with the aid of examples.

Polymer

As polymer, a polypropylene having a melt flow index (MFI) at 230° C./2.16 kg of 1.7 g/10 min was used during the trials.

Each time 300 kg of polypropylene granules were employed, using 25 kg drums.

The polypropylene granules and the optional additive granules were fed directly into the extruder and melted.

Spinning Conditions

Extruder: Diameter: 40 mm; barrel length: L/D = 25 Extruder pressure: 80 bar Flow rate: 14.5 kg/h 5 heating zones Spinning pump: Metered volume: 10 cm³/revolution Spinning unit: Electrically heated Metering pump: 23.19 revolutions/min. Spinnerets: Diameter: 0.7 mm Capillary length: 3 × D Water bath: Distance between spinneret and water bath: 45 mm Temperature: 50° C.

Drawing Stages and Heating Channels

Drawing stage 1: 7 rolls; roll diameter: 230 mm; 1 heating channel Drawing stage 2: 7 rolls; roll diameter: 230 mm; 1 heating channel Drawing stage 3: 7 rolls; roll diameter: 230 mm; 2 heating channels Drawing stage 4: 4 rolls; roll diameter: 230 mm.

Measurement Methods

-   -   Melt flow index according to ASTM D1238     -   Determination of the titre according to SN 197 012 and SN 197         015, supplemented with DIN 53830/     -   The mechanical constant MC is calculated using the following         formula:

MC=√ D×F [cN/tex]

where D denotes the elongation in [%] and F denotes the strength in [cN/tex]

-   -   Determination of the thermal shrinkage according to BISFA         (Chapter 11 of the standard protocol: “Determination of the         thermal shrinkage in hot air”), carried out under the following         conditions:         -   tension: 0.02 cN/dtex         -   temperature: 120° C.         -   duration: 10 min.

Description of the Abrasion Tests

Manufacture of sectional beams

The sectional beams, of 1000 m each, were manufactured using monofilaments from 60 bobbins of the various versions.

Weaving Trials

The weaving trials were carried out on a ribbon loom.

Maximum possible production: 4000 revolutions/min.

The shed was formed by excentrics

Working mode: no weft re-entry

Density of warp yarns: 22.80 yarns/cm

Reed: opening: 0.175 mm

-   -   dent thickness: 0.264 mm     -   dent width: 7.0 mm

Speed of rotation of the loom: 1000 revolutions/min.

Weaving speed: 10 m/h

Weave: L1/1 (plain weave) cloth.

Evaluation of the Abrasion Behaviour

-   -   visual evaluation of the reeds     -   gravimetric determination of the amount of fluff produced.

In the visual determination, the reeds are photographed after a period of operation corresponding to a 300 m length of fabric, and a rating is assigned to them.

FIGS. 1A, 1B and 1C show photographs of the reeds after a weaving trial according to the method described above. FIG. 1A corresponds to Example 1, FIG. 1B to Example 2 and FIG. 1C to the comparative example. The reeds of FIGS. 1A and 1B show practically no fluff, whereas that of FIG. 1C shows a not insignificant amount of fluff.

The evaluation of the abrasion behaviour using the gravimetric method is described below. To do this, all of the fluff formed after a period of operation corresponding to a fabric length of 300 m is collected, weighed and expressed with respect to the warp yarns using the following formula:

${\% \mspace{14mu} {fluff}\mspace{14mu} {deposit}} = \frac{{mass}\mspace{14mu} {of}\mspace{14mu} {deposit} \times 100}{{{No}.\mspace{14mu} {of}}\mspace{14mu} {warp}\mspace{14mu} {yarns} \times \frac{{warp}\mspace{14mu} {yarn}\mspace{14mu} {length} \times {titre}}{10000}}$

The abrasion behaviour may also be evaluated using the same gravimetric method described above, but for a fabric length of 1000 m.

FIG. 2 shows the results in terms of abrasion behaviour of Examples 1 and 2 and the comparative example.

EXAMPLES Example 1

Monofilaments were obtained from granules of polypropylene having a melt flow index (MFI) at 230° C./2.16 kg of 1.7 g/10 min, using a method and under the conditions described above.

Example 2

Monofilaments were obtained from granules of polypropylene and from granules consisting of polypropylene and of PP/PE modified polyolefin, with a melting point >140° C.

The examples are summarized in Table 1.

TABLE 1 Example 1 2 Additive (% by weight) — 10 Diameter [mm] 0.15 0.15 Strength [cN/tex] 61 56.5 Elongation corresponding 35.7 38.5 to the maximum tensile stress [%] Mechanical constant 364 351 Thermal shrinkage (%) 2.5 3.4 Abrasion (%) (300 m of 0.0094 0.004 fabric) Abrasion (%) (1000 m of — 0.0206 fabric)

Comparative Example

Monofilaments were obtained in accordance with Example 2 of Patent Application Wo 03/048434. Their abrasion was 0.0156%.

The monofilaments according to the invention are suitable for the manufacture, without any abrasion, of fabrics intended for filtration.

Thanks to the monofilaments according to the invention, it is possible to weave the polypropylene monofilament practically without any abrasion and to considerably increase the operation time of the loom. This mono-filament is particularly suitable for the manufacture of fabrics that are used for filtration in the chemical, pharmaceutical and food industries. 

1-11. (canceled)
 12. Polypropylene-based monofilaments, made from a polypropylene having a melt flow index (MFI) at 230° C./2.16 kg of less than or equal to 1.9 g/10 min.
 13. The monofilaments according to claim 12, having an abrasion of less than or equal to 0.014%.
 14. The monofilaments according to claim 13, wherein the abrasion value is less than or equal to 0.01%.
 15. The monofilaments according to claim 12, having a strength of at least 50 cN/tex for an elongation corresponding to the maximum tensile stress (elongation at break) at least equal to 20%.
 16. The monofilaments according to claim 12, further comprising 0.01 to 20% by weight of additive relative to the weight of polypropylene.
 17. The monofilaments according to claim 5, wherein the additive is a polypropylene/polyethylene copolymer having a melting point ≧140° C.
 18. The monofilaments according to claim 6, wherein the copolymer is present in a quantity of 1 to 20% by weight of this copolymer relative to the weight of polypropylene.
 19. A process for manufacturing the monofilaments as defined in claim 12, comprising the steps of extrusion/melt-spinning, drawing and winding of the polypropylene.
 20. A two-dimensional high-performance article comprising the monofilaments as defined in claim
 12. 21. A fabric for filtration comprising the monofilaments as defined in claim
 12. 